Imperial Metals’ estimate of the size of the spill from its Mount Polley mine tailings dam collapse is nearly 70 per cent greater than the initial estimate.

The B.C. government has estimated that 10 million cubic metres of water and 4.5 million cubic meters of finely ground rock containing potentially-toxic metals was released by the collapse of the dam on Aug. 4.

But Imperial Metals has estimated the size of the spill at 10.6 million cubic metres of water, 7.3 million cubic metres of tailings and 6.5 million cubic metres of “interstitial” water. That’s enough water and material to fill nearly 9,800 Olympic-sized swimming pools.

Interstitial water is the water suspended in the spaces between the finely ground rock of the tailings.

“It’s a bit disconcerting — its speaks to the crudeness of the initial estimate,” said Mining Watch Canada program director Ramsey Hart of the increased spill estimate.

Imperial Metals did not respond to a request Wednesday for comment.

Hart said there will need to be a better accounting of the spill’s size, including the volume of tailings deposited in the lake and in the Hazeltine Creek watershed.

The early numbers were best estimates, later refined by Imperial Metals, said B.C. Mines Ministry spokesman Ryan Shotton in a written statement.

“The company is undertaking work to fully characterize the chemistry of the tailings and water that have been released. This work will include testing for a full suite of parameters, including metals,” the statement said.

University of B.C. mining engineering professor Scott Dunbar said he didn’t believe there would be a significant difference in the composition of the interstitial water and the water above the tailings in the storage facility. The company has said the water above the tailings was near drinking-water quality.

And Dunbar said while the tailings certainly have been physically damaging, the science suggests they will not be chemically damaging.

That’s because the tailings are not acid generating, noted Dunbar, the head of UBC’s Norman B. Keevil Institute of Mining Engineering.

Acid-generating rock is a problem for the environment because the sulphides in rock release the metals in minerals trapped in the rock, he explained.

“But they have to go through every square metre, or square five metres (of the spilled tailings), and check to see if this in fact is the case,” added Dunbar. “And if that is the case, and it’s not damaging, then the cleanup could be reshaping the landscape, reseeding. In the extreme, it’s removal. And that will be expensive.”

Hart said he would also like to see more reporting on the geochemistry of the tailings materials and on the potential for metal to leach from them.

Several test results of the sediments within the tailings storage facility and outside of it have shown low but potentially significant arsenic and selenium concentration, the province has said.

Hart also said, however, he believed the chemistry of the interstitial water could be different from that of the water above the tailings.

“The water has been in contact for different amounts of time — and the fact that there are low sulphides doesn’t necessarily mean there won’t be metal leaching because things like selenium, arsenic and zinc all come out of mine wastes without the acid rock draining that Imperial Metal keeps saying won’t happen,” said Hart.

The rush of water and tailings scoured Hazeltine Creek and poured the water and some of the tailings into Quesnel Lake. Coho salmon and rainbow trout spawn in Hazeltine Creek. Some of the water and tailings also entered Polley Lake, a small lake adjacent to the mine site.

While provincial water tests continue to show the water in Quesnel Lake meets drinking guidelines, the province has warned residents not to drink cloudy water. Residents have reported a plume of sediments in the lake that shows up and disappears. Tests have shown some elevated levels of metals that exceed drinking and aquatic life guidelines in water containing sediments.

Residents, First Nations and environmental groups also say they are concerned about the long-term effects of the spill, including on salmon.

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